The present invention relates to a method for preparing glass fibers and in particular to the application of a forming size to continuous filament glass fiber strand.
Glass fibers are produced by a means whereby a molten glass composition is flowed or pulled through tiny orifices or tips in a heated platinum bushing. The individual glass filaments are passed through a sizing bath, grouped into a strand and then wound on a rapidly rotating forming tube. A size is applied to the filaments in order to bond them together, maintain the integrity of the strand during winding and unwinding as well as facilitate eventual processing. The strand on the forming tube is thereafter placed in an oven to dry or is allowed to air dry to reduce the moisture content of the strand.
There are many different compositions which have been in use as glass forming sizes. Typically the sizes have comprised aqueous dispersions of various modified starches and oils. The following patents are directed to such compositions:
U.S. Pat. No. 3,227,192 (issued Jan. 4, 1966 to D. Griffiths) discloses an aqueous forming size containing an amylose starch mixture containing approximately equal portions of a high amylose starch component having an amylose content of about 50-60% and a low amylose starch component having an amylose content of about 20-30%.
U.S. Pat. No. 3,167,468 (issued Jan. 26, 1965 to J. Lovelace et al.) discloses an aqueous forming size containing a starch ether or ester which is the reaction product of starch and a tertiary or quaternary amine.
U.S. Pat. No. 3,481,771 (issued Dec. 2, 1969 to A. Doering) discloses an aqueous forming size dispersion having a low metallic ion content which employs an inhibited (crosslinked) or unhibited starch ether or ester derivative. Among the applicable starch esters listed are acetates, propionates, butyrates, laurates, stearates and oleates. Suitable crosslinking agents listed are aliphatic dihalides, ether forming epoxy halogen compounds (i.e., epichlorohydrin), polyfunctional reagents (i.e., phosphorus oxychloride), mixed anhydrides, and succinic anhydride.
U.S. Pat. No. 3,615,311 (issued Oct. 26, 1971 to R. Ignatius) discloses a forming size containing a cationic starch ether or ester and an underivatized starch that is preferably high in amylose.
U.S. Pat. Nos. 3,664,855 and 3,793,065 (issued May 23, 1972 and Feb. 19, 1974, respectively to A. Morrison et al.) employ starch ethers and esters of a substituted or unsubstituted monocyclic 5 or 6 member hydrocarbon ring in a glass forming size composition.
U.S. Pat. No. 3,928,666 (issued Dec. 23, 1975 to Morrison et al.) employs a starch ester of a 4-6 carbon isoalkanoic acid in a glass forming size composition.
U.S. Pat. No. 4,166,872 (issued Sept. 4, 1979 to R. Karpik et al.) is directed to a migration resistant forming size which contains a portion of swollen but unburst partially cooked starch granules preferably from a high amylose corn hybrid.
U.S. Pat. No. 4,168,345 (issued Sept. 18, 1979 to B. de Massey et al.) is directed to a glass forming size composition containing both a modified starch of low viscosity and an acrylic polymer film forming agent. The useful starch is modified by treatments including etherification, esterification, cationization or acid hydrolysis.
One problem addressed by many of the above patents is that of size migration, a phenomenon which occurs in the strand while it is wound on the forming tube. As the strand is dried, water moves from the interior of the forming tube to the exterior carrying with it some of the starch and oil of the size. The uneven deposition of starch on the fibers creates a number of problems in subsequent processing of the strands. As the strands are wound and unwound during various operations, tension of the strand should remain relatively constant. Uneven tension, created by non-uniform size distribution, causes individual glass filaments of the strand to become broken resulting in fuzzy strands which can clog fabrication equipment. Moreover, heat treatment of the fibers after fabrication, typically conducted at temperatures of about 650.degree. C. in order to volatilize the size solids, results in uneven and unsatisfactory size removal. The residue remaining after thermal desizing will alter the dielectric properties of the fabricated piece and cause streaking or local discoloration.
The handling characteristics of sized glass fibers including the ability of the fibers to be processed at high speeds is related to the glass strand integrity. A glass strand with excellent strand integrity will exhibit a minimum number of broken filaments as well as minimum filament separation from the main strand after the winding and twisting processes subsequent to formation. In order to produce a glass strand having good strand integrity, a size formulation having good film forming properties is advantageously employed. While a high level of non-ionic oil in the glass size formulation is essential for lubrication during the formation operation, it is known to have a deleterious effect on the film formation of a starch size. As such, starch film integrity is usually sacrificed in order to provide for the economical production of the glass fiber.
The glass forming industry is still searching for means for providing sized glass fibers which exhibit minimal size migration and improved strand integrity.
Accordingly, it is an object of the present invention to provide an aqueous size which can be applied to glass fibers upon formation which will exhibit minimal size solids migration upon drying. It is also an object to provide an aqueous glass forming size which will provide good glass strand integrity.
None of the references mentioned above disclose or suggest the glass forming size compositions of the present invention.